Ionization gauges, more specifically Bayard-Alpert (BA) ionization gauges, are the most common non-magnetic means of measuring very low pressures and have been widely used worldwide since being disclosed in U.S. Pat. No. 2,605,431 in 1952.
A typical ionization gauge includes an electron source, an anode, and an ion collector electrode. For the BA gauge, the electron source is located radially outside of an ionization space (anode volume) defined by the anode. The ion collector electrode is disposed within the anode volume. Electrons travel from the electron source toward and through the anode, and are eventually collected by the anode. However, in their travel, the electrons impact molecules and atoms of gas, constituting the atmosphere whose pressure is to be measured, and create ions. The ions are attracted to the ion collector electrode by the electric field inside the anode. The pressure of the gas within the atmosphere can be calculated from ion and electron currents by the formula P=(1/S) (Iion/Ielectron), where S is a constant with the units of 1/torr and is characteristic of a particular gauge geometry and electrical parameters.